1. Field of the Invention
The subject invention relates to projection television receivers, and more particularly, to cooling liquids for cooling a projection cathode ray tube in the projection television receiver.
In a cathode ray tube, a field of video information is written with the aid of an electron beam on a display screen having a phosphor coating or a pattern of different phosphors. Due to the electron bombardment, the temperature of the phosphor increases so that the light output of the display screen decreases ("thermal quenching"). While this is acceptable in conventional direct-view television receivers, this phenomenon is particularly prevalent in display tubes for projection television receivers in which the display screen is scanned by electron beams having high fluxes. The temperature of the display window increases and brings about a temperature gradient which causes a mechanical stress in the display window. At a high electron beam current and consequently a high thermal load, this may lead to breakage of the display window. To reduce this mechanical stress in the display window due to variations in temperature ("thermal stress") and to obviate the decrease in light output, the display window and the display screen are cooled.
2. Description of the Related Art
U.S. Pat. No. 4,725,755 to Hasegawa, discloses a projection television receiver with a liquid-cooled lens. The projection television receiver includes at least one cathode ray tube for receiving video signals and for projecting images therefrom, having a face, a lens mounted adjacent to the face for focusing the images projected by the cathode ray tube, and a coolant disposed between the lens and the face for absorbing and dissipating heat from the tube. The coolant is particularly specified as being a mixture of glycerol and ethylene glycol including about 20 to 40% by weight of glycerol. This coolant differs from coolants previously used in that glycerol is used instead of water. This substitution reduces the amount of vapor pressure at elevated temperatures. However, it should be noted that ethylene glycol is hygroscopic, i.e. it absorbs water from the atmosphere. Therefore, care must be taken to limit the exposure of the ethylene glycol to moisture. In addition, surfactants should be added to the ethylene glycol/glycerol mixture to reduce the surface tension, along with oxidizers to increase the stability of the mixture.